The immune system is composed of many interdependent cell types that collectively protect the body from bacterial, parasitic, fungal, viral infections and from the growth of tumour cells. The guards of the immune system are macrophages that continually roam the bloodstream of their host. When challenged by infection or immunisation, macrophages respond by engulfing invaders marked with foreign molecules known as antigens. This event, mediated by helper T cells, sets forth a complicated chain of responses that result in the stimulation of B-cells. These B-cells, in turn, produce proteins called antibodies, which bind to the foreign invader. The binding event between antibody and antigen marks the foreign invader for destruction via phagocytosis or activation of the complement system. A number of different classes of antibodies, also known as immunoglobulins, exist, such as IgA, IgD, IgE, IgG, and IgM. They differ not only in their physiological roles but also in their structures. From a structural point of view, IgG antibodies have been extensively studied, perhaps because of the dominant role they play in a mature immune response. Polyclonal antibodies are produced according to standard methods by immunisation of an animal with the appropriate antigen. In response, the animal will produce antibodies which are polyclonal. However, for many purposes, it is desired to have a single clone of a certain antibody, known as monoclonal antibodies. Monoclonal antibodies (MAbs) are produced by hybrid or fused cells comprised of a fusion between a normal B-cell, which produces only a single antibody, to an abnormal myeloma tumour cell. The resulting hybrid, known as a hybridoma, is these days used in standard methods for the production of antibodies.
The biological activity that the immunoglobulins possess is today exploited in a range of different applications in the human and veterinary diagnostic, health care and therapeutic sector. In fact, in the last few years, monoclonal antibodies and recombinant antibody constructs have become the largest class of proteins currently investigated in clinical trials and receiving FDA approval as therapeutics and diagnostics. Complementary to expression systems and production strategies, efficient purification protocols are required to obtain highly pure antibodies in a simple and cost-efficient manner.
Traditional methods for isolation of immunoglobulins are based on selective reversible precipitation of the protein fraction comprising the immunoglobulins while leaving other groups of proteins in solution. Typical precipitation agents are ethanol, polyethylene glycol, lyotropic salts such as ammonium sulphate and potassium phosphate, and caprylic acid. Typically, these precipitation methods are giving very impure products while at the same time being time consuming and laborious. Furthermore, the addition of the precipitating agent to the raw material makes it difficult to use the supernatant for other purposes and creates a disposal problem, which is particularly relevant when speaking of large-scale purification of immunoglobulins.
An alternative method for isolation of immunoglobulins is chromatography, which embraces a family of closely related separation methods. The feature distinguishing chromatography from most other physical and chemical methods of separation is that two mutually immiscible phases are brought into contact wherein one phase is stationary and the other mobile. The sample mixture, introduced into the mobile phase, undergoes a series of interactions with the stationary and mobile phases as it is being carried through the system by the mobile phase. Interactions exploit differences in the physical or chemical properties of the components in the sample. These differences govern the rate of migration of the individual components under the influence of a mobile phase moving through a column containing the stationary phase. Separated components emerge in the order of increasing interaction with the stationary phase. The least retarded component elutes first, the most strongly retained material elutes last. Separation is obtained when one component is retarded sufficiently to prevent overlap with the zone of an adjacent solute as sample components elute from the column. Efforts are continuously being made to design the optimal stationary phase for each specific separation purpose. Such a stationary phase is commonly comprised of a support or base matrix to which a ligand comprising functional i.e. binding groups has been attached. Reference is commonly made to each kind of chromatography based on the principle of interaction it utilises, such as affinity chromatography, hydrophobic interaction chromatography and ion-exchange chromatography.
Affinity chromatography is based on specific interactions between a target biomolecule and a biospecific ligand according to a principle of lock-key recognition. Thus, the target and ligand will constitute an affinity pair, such as antigen/antibody, enzyme/receptor etc. Protein-based affinity ligands are well known, such as Protein A and Protein G affinity chromatography which are both widespread methods for isolation and purification of antibodies. It is well known that Protein A chromatography provides an outstanding specificity, particularly towards monoclonal antibodies, and consequently high purities are obtainable. Used in combination with ion exchange, hydrophobic interaction, hydroxyapatite and/or gel filtration steps, Protein A-based methods have become the antibody purification method of choice for many biopharmaceutical companies, see e.g. WO 8400773 and U.S. Pat. No. 5,151,350. However, due to the peptide bonds of the proteins, protein A matrices present a certain degree of alkaline sensitivity. In addition, when Protein A matrices are used to purify antibodies from cell culture media, proteases originating from the cells may cause leakage of Protein A, or peptide fragments thereof.
Ion exchange chromatography is frequently used in protocols for the isolation of immunoglobulins. In anion exchange chromatography, negatively charged amino acid side chains of the immunoglobulin will interact with positively charged ligands of a chromatography matrix. In cation exchange chromatography on the other hand, positively charged amino acid side chains of the immunoglobulin will interact with negatively charged ligands of a chromatography matrix.
Hydrophobic interaction chromatography (HIC) is another method described and used in protocols for the isolation of immunoglobulins. If a highly pure immunoglobulin product is the object, it is commonly recommended to combine HIC with one or more further steps. In HIC, in order to make the immunoglobulin bind efficiently to the HIC matrix, addition of lyotropic salts to the mobile phase is required. The bound immunoglobulin is subsequently released from the matrix by lowering the concentration of lyotropic salt. Thus, a disadvantage of this procedure is the necessity to add lyotropic salt to the raw material, as this may cause problems and a consequently increased cost to the large-scale user. For example, for raw materials such as whey, plasma, and egg yolk, the addition of lyotropic salts to the raw materials would in many instances be prohibitive in large-scale applications, as the salt could prevent any economically feasible use of the immunoglobulin depleted raw material. An additional problem in large-scale applications would be the disposal of several thousand litres of waste.
U.S. Pat. No. 5,945,520 (Burton et al) discloses mixed mode chromatographic resins which exhibit a hydrophobic character at the pH of binding and a hydrophilic and/or electrostatic character at the pH of desorption. The resin is specifically designed to bind the target compound from an aqueous solution at both a low and high ionic strength. This is achieved by selected ionisable ligands comprising a spacer arm and at least one ionisable functionality, wherein the density of the ionisable ligands on the solid support matrix is greater than the smaller of either about 150 μmol/mL resin or 1 mmol/gram dry weight of resin. In addition, the hydrophobic character of the resin comprising said ionisable ligands is sufficient to bind at least 50% of the target compound in an aqueous medium at high and low ionic strength at a first pH. Illustrative examples of the ionisable functionality are 4-(aminomethyl)pyridine, 3-(aminomethyl)pyridine, 2-(aminomethyl)pyridine, 1-(3-aminopropyl)-imidazole, 2-(aminomethyl)-benzimidazole, 4-(3-aminopropyl)morpholine.
Further, WO 01/38228 (Belew et al.) relates to a method for anion-exchange adsorption wherein thioether anion-exchangers are used to remove a negatively charged substance from a liquid by binding thereof. Each ligand comprises a positively charged nitrogen and a thioether linkage at a distance of 1-7 atoms from said charged nitrogen. The desired substances, such as cells, parts of cells and substances comprising peptide structures are adsorbed at salt concentrations in the region of 0.25M NaCl.
Finally, U.S. Pat. No. 6,702,943 (Johansson et al) discloses a method for removal of a target substance from a liquid by adsorption thereof to a matrix carrying a plurality of ligands comprising anion-exchanging groups and a hydrophobic structure. More specifically, the ligands contain an aromatic ring in the proximity of the positively charged anion-exchanging groups. The desired substances are stated to be cells, parts of cells and substances comprising peptide structures. The ligands disclosed are denoted “high salt ligands” due to their capability of adsorbing target substances at high concentrations of salt such as 0.25M NaCl.
However, to optimise a process related to the purification of a specific target molecule, unique operating conditions will be required, and the best separation matrix will vary from case to case. For example, in the biotech industry, specific processes need to be designed for the purification of peptides and proteins; nucleic acids; virus etc. Further, in the purification of antibodies, the type of antibody will decisive for the choice of separation matrix. Thus, there is still a need in this field of alternative separation matrices to provide a broad spectrum of choices for the purification of the many new products that are constantly developed.